i like to put a small led light on the hot side of the glow plugs so i can see if they are getting power.
fuel delivery on the mechanical lift pump is rated at 7 psi and 1/3 pint in 10 seconds starter cranking time. if you have that your fuel system is fine.

before you use the ether to fire it, what happens?
an easy sign of air intrusion is crank, start and run for a few seconds, then die and take forever to restart.

another easy way to tell if you are getting air intrusion is to pll the fuel filter off when cold. if it is half empty, you have air intrusion. if it is full, you do not.
i wonder if you have low compression maybe? if you are under 300 lbs in all cylinders that would cause a hard cold start/only start with ether condition.

 

Make sure power is getting to the GP's. I had to make a new GP harness for mine. I still believe you have air intrusion. New hoses and o-rings on the return lines would make a huge difference. Sounds like you are running the GP's manually. The GP controller runs them a bit more than just until the wait to start light goes out. It pulse them through the starting phase and if really cold pulses a bit after it starts. It is heating the air and fuel in the precup.

 

Would anyone suggest installing the controller for the GPs available from accuratediesel.com ?

Ford 6.9L IDI Controller

I figure at that price, it's good place to start (or hopefully start? )

 

that is the old style controller used with the ZD-1 plugs. it is the one that likes to stick on and burn out the glow plugs.
your truck will have the new style solid state controller used on the 88 and newer trucks.